[Haskell-beginners] How to process a list that contains "undefined" values?

Hi Folks, How would you find the maximum value in this list: [undefined, 10, undefined, 20] Is it bad practice to create lists that contain undefined values? If yes, what is a better way to express in a list a "no value present/available" value? For example would it be better to express the above list using Maybe: [Nothing, Just 10, Nothing, Just 20] Or perhaps something else? What do you recommend? /Roger